Multiple-input multiple-output (MIMO) is a radio communication technology that exploits multiple transmitters and receivers to transfer data at the same time. MIMO systems can achieve a large throughput via the ample spatial degree of freedoms provided by a large amount of antennas. MIMO has become an essential element of wireless communication standards, such as IEEE 802.11n (Wi-Fi), IEEE 802.11ac (Wi-Fi), and Long Term Evolution (4G), and will also play a major role in 5G network deployments.
To achieve these performance advantages, the transmitter requires instantaneous channel state information (CSI), which can be represented as a vector that contains a large number of elements changing on a fast timescale. In frequency division multiplexing (FDM) systems, the CSI is generally available at the user communication devices and, hence, the user communication devices need to feed the CSI back to the transmitter (e.g., a base station) at every timeslot. Based on the global CSI feedback, the base station computes precoders for all the user communication devices by exploiting spatial multiplexing and by means of these precoders can serve multiple user communication devices simultaneously. This feedback scheme will be referred to herein as “CSI feedback scheme”. However, the CSI feedback scheme in multiuser MIMO is challenging, because the uplink resource is limited, and the number of user communication devices to be served opportunistically can be large. In consequence, each user communication device may be allocated only a small number of bits for the CSI feedback.
In parallel to cellular networks, device-to-device (D2D) communication is a fast developing technology that allows user communication devices to directly communicate with each other without having to route the communication via a base station. D2D communication can be implemented in an in-band mode or an out-band mode according to different requirements on spectrum efficiency, quality-of-service, interference level, complexity of scheduling, resource allocation and the like. It is possible that user communication devices can share the CSI among themselves via D2D communication to improve the downlink/uplink transmission in the cellular network provided by a base station.
Several previous works (“Precoder Feedback versus Channel Feedback in Massive MIMO under User Cooperation” by J. Chen, H. Yin, L. Cottatellucci, D. Gesbert, Proc. Asilomar Conf on Signals, Systems, and Computers November 2015; “Enabling massive MIMO systems in the FDD mode thanks to D2D communications” by H. Yin, L. Cottatellucci, D. Gesbert, Proc. Asilomar Conf. on Signals, Systems, and Computers, Pacific Grove, Calif., November 2014; “Advanced massive MIMO algorithms, Phase 2 report” by H. Yin, J. Chen, L. Cottatellucci, and D. Gesbert, 2015) have studied the application of a D2D communication system to the feedback and precoder design of MIMO cellular networks. It has been shown that, in an ideal case, wherein user communication devices have infinite or perfect D2D communication capability and share their CSI perfectly between each other, it is better to return the precoder rather than the CSI as feedback to a base station. More specifically, each user communication device can compute the precoder itself, since the perfect global CSI is available at each user communication device. It has been demonstrated that, when the user communication devices have a very limited number of bits for the feedback to the base station, the precoder feedback scheme significantly outperforms the CSI feedback scheme.
However, practical implementations of the above theoretical studies face several critical problems. Firstly, high-quality D2D communication is not easy to implement in a realistic system. In practice, user communication devices often have only a limited D2D communication capability. Secondly, it is difficult to compute the turning point (i.e., the critical number of feedback bits available in the system) and to determine when to switch from the CSI feedback scheme to the precoder feedback scheme, or vice versa. Intuitively, D2D CSI sharing with limited rates should still benefit the system, but the current existing schemes fail to exploit that. Thirdly, existing methods are not compatible with the CSI feedback scheme. More specifically, they cannot work in a system, wherein some users have D2D links and perform precoder feedback to the base station, while some others have no D2D links at all and hence have to perform CSI feedback to the base station.
Firstly, generally the CSI exchange among users is not perfect. For example, when the D2D communication quality is poor, the users can only transmit signals with a low data rate over D2D system, and the CSI to be exchanged needs to be quantized using a small number of bits, resulting in a large distortion. As a result, the desired feedback and precoding strategy should be robust to the CSI noise due to the limited D2D CSI exchange. Secondly, the users may experience heterogeneous D2D quality, wherein some user pairs may have good D2D quality, while some other user pairs may have poor D2D quality or no D2D communication at all.
Thus, in light of the above there is a need for a more practical and flexible user communication device and base station as well as corresponding methods, which, in particular, allow improving MIMO cellular communications under varying D2D communication capabilities.